1. Field of the Invention
This invention relates to a method and apparatus for identifying radar targets whether fixed or mobile. The method and apparatus are especially useful for identifying ships at sea.
2. Prior Art
At present, ships at sea can only be identified by visual observation of the ship's name and port of registry which are painted on the hull, or by observation of flags or lamp signals indicating the four-letter International Identification Code signal of the vessel. These means of identification are inadequate at night or in bad visibility. There is accordingly a need for an improved method of identifying ships at sea, and since many observations of ships are today effected by radar rather than visually, it is advantageous for such a method to employ a radar-triggered identification signal. The present invention provides such an identification method and apparatus for use therein.
Radar identification systems have been known for many years, having originated during the Second World War in the IFF (Identity Friend or Foe) system. In such systems, an interrogation apparatus sends out a pulse of radio waves (usually of about radar frequency), which is recognised by an identification apparatus (known as a responder or transponder) fitted to a vehicle or other object within range of the interrogation apparatus. Upon receipt of the pulse, the responder sends back a reply signal, generally in the form of a train of pulses, which is decoded and displayed at the station containing the interrogation apparatus. Such systems, which require the provision of a responder on the vehicle or other object, are known as secondary radars, as distinct from primary radars which work by passive reflection of radio waves by targets. Normally a secondary radar is associated with a primary radar so that the responses received from responders on radar targets can be correlated with the echoes received by the primary radar from the targets themselves. Secondary radar systems are at present in use on some navigation buoys (the so-called "RACONS") and in the Air Traffic Control Responder (ATCR) system. In the latter system, the pilot of an aircraft, upon request by Air Traffic Control, sets his responder to give a desired 16-bit upon receipt of the appropriate radar signal; this 16-bit response is transmitted over a period of about 15.mu. secs.
The ATCR system cannot, in practice, be adapted to marine use. An airline pilot is trained in the use of the system and is in continual contact with Air Traffic Control. The minimum separation between aircraft is 1 nautical mile (n.m.) and is usually considerably greater, so that the apparatus has only, at most, to distinguish between two targets 1 n.m. apart. Air Traffic Control normally knows what aircraft should be within its airspace and can ask the pilots thereof to set their responders as Air Traffic Control desires. In addition, differences in height aid the identification of aircraft.
In contrast, the crews of ships at sea are not trained in the use of secondary radar systems and are not in continual contact with shore radar stations. They may approach each other more closely than aircraft and a practical primary radar system should be capable of distinguishing between two targets 0.5 n.m. apart on the same bearing. A shore station usually does not know what ships likely to be in its vicinity at any one time, and due to reflections from waves the problem of radar clutter is more serious in systems intended for use with ships than in systems intended for aircraft.
Because ship's crews are not trained in the use of secondary radar systems, it is in practice essential that the responder of a maritime secondary radar system should be such that once set, it will function automatically for long periods without attention from the crew. Thus, each ship must carry a responder giving a unique signal. To provide the necessary number of different responses, the response must take the form of a pulse train containing a considerable number of different binary digits (bits); for ease of reference, the response should indicate the four-letter International Identification Code signal of the vessel (which requires 24 bits) or, preferably, since not all nations' fleets use International Identification Codes, the seven-figure Lloyds' number (which requires 28 bits). However, the time between the receipt at the interrogator of the responses from two targets 0.5 n.m. apart on the same bearing is only about 6.mu. secs. and to avoid garbling (the corruption of signals due to signals from two different targets arriving at the interrogator at the same time), it is essential that the two responses, as received at the interrogator, should not overlap. Accordingly, the duration of the signal produced by the responder should not exceed about 6.mu. secs. To transmit at the responder 24 or 28 bits within 6.mu. secs. and to receive and decode with an acceptable error rate at the interrogator, under reception conditions which may be far from favourable, would make such great demands upon the bandwidth, timing and transmission power of the responder as to make that responder too complex and expensive for the system to be practicable.
The present invention seeks to provide a method for identifying a radar target which permits the responder to transmit a large number of bits concerning its identity using an acceptable bandwidth and pulse length. The present invention also provides an interrogator and responder for use in this method.